The present invention relates to feedback control systems for controlling the position of a working valve or the like. More specifically, the present invention is directed to a pneumatically actuated and hydraulically driven control system.
Feedback control systems employed for the actuation and positioning of a working valve or the like have included pneumatic systems, hydraulic systems and electro mechanical systems. Each of these types of systems have particular advantage under some conditions. However, each of these types of systems also exhibit characteristic practical deficiences.
Pneumatic systems are convenient in the sense that many industrial plants, refineries and power plants distribute pressurized shop air for power equipment and control signals. Pneumatic systems do not require return lines, are generally fire safe and the supply is easily distributed. However, pneumatic systems often experience dynamic instability because of the compressible nature of the control and working fluid. Furthermore, with many systems, volumetric efficiency is relatively low resulting in power inefficiencies and extended response times. The compressible nature of the pneumatic fluid does allow faster movement of components under a low load condition; however, once a high load is experienced, time is lost building the needed pressure in a large cavity. Pneumatic pressures are also normally kept low relative to hydraulic pressures in comparable hydraulic systems. This increases the size of the pneumatic system because of the additional work area required to offset the low pressure.
Hydraulic systems on the other hand do not suffer from compressible fluid dynamic instability. Even with a hydraulic system shut down where only low pressures remain in hydraulic cylinders, accurate retention of driven components is possible, again because of the incompressible nature of the fluid. Furthermore, high pressures can be employed which reduces the size of the operating equipment. However, hydraulic fluids are often not fire proof and hydraulic systems are notorious for leakage and high maintenance, particularly in control applications. Hydraulic systems employing proportional devices can experience overheating of the hydraulic fluid, fluid breakdown, and eventual destruction of components because of high pressure loss from internal leakages through small orifices under low demand and static conditions. The ability of the hydraulic fluid to carry peak pressures and shocks throughout the hydraulic system often also results in component failure. Thus, like pneumatic systems hydraulic systems have characteristic advantages and disadvantages inherent in the nature of the fluid and the apparatus.
Electro mechanical as well as electro pneumatic and electro hydraulic systems also characteristically exhibit advantages and disadvantages. The electrical components associated with such systems are often subject to electronic interference. Electrical systems are often subject to sparking which can, in many circumstances, create a very hazardous condition from fire. With mechanical systems, motor drives react slowly, mechanical springs change spring rate with use and bearings wear while in hydraulics and pneumatics these conditions can be avoided. Failure modes for mechanical systems generally requires a significant amount of additional equipment depending on the failure mode selected. It can be seen from the foregoing, that many disadvantages are inherent in the various types of feedback control systems; and the type of system is often selected to avoid the disadvantages of another, otherwise better suited type of system.